While a check card purchase is immediately reflected in a checking account's available balance, it doesn't necessarily stay there. Swiping the card creates an authorization hold which temporarily decreases the balance by a certain amount. This is mainly to verify that the account is open, and that there are funds available at the time. The hold lasts for one business day. When the hold expires the transaction is no longer reflected in the available balance until the merchant presents the receipt to the bank. They can take their time with this, but typically it takes two or three business days. Once the receipt is presented (electronically), it posts to the account for the amount written on the receipt. To a merchant, it looks just like a credit card (except for maybe saying "Check Card" on it).

Authorizations and posted transactions aren't necessarily a one-to-one relation. If you get a refund for a transaction that hasn't posted yet, the merchant may just not present the receipt rather than presenting it and then crediting it back. Merchants can also post transactions on a recurring basis. At least at the bank where I work, check card payments can't be stopped from posting to an open account once they're authorized, unless the transaction is a recurring payment for the same amount. Also, because they are guaranteed by Visa (for check cards that are Visa), check card charges are always paid. Whether you can get unauthorized charges credited back once they're paid depends on your bank.

It's worth noting that the authorization hold doesn't have to be for the amount that will be charged. Gas stations almost always authorize $1.00. Restaurants authorize a certain percentage over the price of the meal to leave room for a tip. Employees may or may not be aware that their card reader does this. As a customer service representative for a major financial institution, I see a lot of overdraft fees caused by the misconception that the charge will immediately come out of their account for the exact amount of the purchase, and stay out (see debit card). When they check their balance after the authorization falls off, it looks like they've got more money than they do. In my opinion, check cards should be better explained before they are issued, regardless of whether the revenue from overdraft fees is greater than the cost of customers calling Customer Service to yell about overdraft fees. It's explained in the depositor's agreement, but who's going to read that? I sure didn't.

To add to the confusion, check cards can also be used as debit cards.